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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Old style lunar
From: Ken Muldrew
Date: 2004 Dec 9, 13:29 -0700
From: Ken Muldrew
Date: 2004 Dec 9, 13:29 -0700
In the "No Lunars Era" thread, Alex was wondering about the precision of lunar distances, among other things, and whether that played a role in navigators not putting much faith in their lunar calculations. While talking about land navigation from the same era, I offered to provide a sample of actual lunar longitudes as an example of the capabilities of the art at that time. On 6 Dec 2004 at 21:21, Alexandre Eremenko wrote: > On Mon, 6 Dec 2004, Ken Muldrew wrote: > >> If you like, I can provide you with some land based lunars taken by >> explorers in the Canadian West ca. 1800. > > Yes. Sure. I would like to see this. Below are a series of latitudes (by meridian altitude of the sun) and longitudes (by lunar distance) that David Thompson took at Rocky Mountain House in 1800 and 1801. He used a sextant by Dollond (9" arc) and a common pocket watch. The true position of Rocky Mountain House is given at the bottom where Thompson's readings are averaged. Latitude: 1800 9-Apr 52?21'29" 1801 20-Feb 52?21'27" 21-Feb 52?21'35" 5-Mar 52?21'32" Longitude (from lunar distance): 1800 17-Apr 115?12'00" 18-Apr 114?57'45" 22-Dec 115?11'00" 1801 17-Feb 114?57'15" 28-Feb 114?52'15" 28-Feb 114?59'45" 1-Mar 115?11'00" 18-Mar 114?44'15" 17-Feb 114?39'00" 24-Feb 114?36'00" 24-Feb 114?13'00" 25-Feb 114?28'30" 25-Feb 114?26'45" Thompson's average position: 52?21'30" 114?48'20" true position of Rocky Mnt. House: 52?21'20" 114?58'50" You can see that the spread of lunars covers a full degree but his final position was pretty close. As far as I have been able to see in his journals, Thompson always updates his account when he takes a latitude or longitude reading. In addition, he updates all the entries in his account log proportionally since the last reading to correct for a systematic bias in his reckoning. He took this latter step because he was intending to map everywhere he travelled, but he clearly put more faith in his celestial observations than did many of the ocean navigators that Frank has written about. Ken Muldrew.